[Python-Dev] PEP 463: Exception-catching expressions

Glenn Linderman v+python at g.nevcal.com
Fri Feb 28 10:30:43 CET 2014

On 2/28/2014 12:41 AM, Chris Angelico wrote:
> On Fri, Feb 28, 2014 at 6:38 PM, Glenn Linderman <v+python at g.nevcal.com> wrote:
>> Whereas the current PEP syntax has ambiguity regarding how to interpret
>> a-expr except except-list-b: b-expr except except-list-c: c-expr (does the
>> 2nd except apply to a-expr or b-expr?), without parentheses, and, as far as
>> I am concerned, even with the parentheses, this syntax makes it very clear
>> that each of the Exception-lists apply to a-expr.
> Fair enough. It's a bit hard to talk about multiple except
> expressions, though, as they're a problem unless formally supported -
> and they're almost never needed. Really, all you need to do is say
> "never abut except-expressions without parentheses" (which the current
> proposal and the "parens around the exception bit only" proposal both
> enforce), and then there's no problem. I expect that normal use of
> this won't include any form of chaining. Yes, it can - like any
> feature - be used abnormally, but at some point it's better to just
> drop it out as a statement.
>> Key advantage to others may be that because the : is within the () [and the
>> leading ( is quite nearby, making it obvious], it is less likely to be
>> considered a statement boundary, and more easily explained as a special type
>> of list syntax... not _really_ a list, because it is really code to be
>> executed somewhat sequentially rather than data, and lists don't have : ...
>> and not _really_ a dict constant, which does have :, because the Exception
>> is not _really_ a key, but the syntax can draw on analogies with the dict
>> constant syntax which will help people remember it, and even sort of
>> understand that there is a pair-wise relationship between the Exception-list
>> and the expression after the :, without repeating the except over and over.
> See the confusing terminology we have here? It might be called a
> "list" of except-expressions, but the brackets are round, and a list's
> are square. It's kinda like dict syntax, only again, the other sort of
> bracket, and it's wrong to try to construct a dict; it'd be too
> tempting to conflate this with some of the other current proposals for
> lazily-evaluated expressions (aka "simpler syntax for lambda" or other
> terms). This is, fundamentally, a multi-part expression on par with
> "and" and "or": first, evaluate the primary expression; then, if an
> exception is raised, evaluate the exception list and see if it
> matches; then, if it matches, squash the exception and evaluate the
> default expression. You can't turn that into a dict, partly because
> you'd need to sort out lazy evaluation, and partly because a dict is
> unordered - if this is expanded to support multiple except clauses,
> they have to be processed in order. (You might, for instance, catch
> ZeroDivisionError, and then Exception, with different handling. It'd
> be VERY confusing for them to be processed in the wrong order,
> particularly if it happens unpredictably.)

That's why I kept saying "not _really_" :)  It isn't a list, but it has 
an order requirement; it isn't a dict, but it has pairs; and finally, it 
isn't data, but code.  But nonetheless the analogies are somewhat useful.

> Are there any other expressions that allow parens around a part of the
> expression, without the stuff inside them becoming a completely
> separate sub-expression?

Sure. Function invocation.  You can claim (and it is accurate) that the 
stuff inside is somewhat independent of the actual function called, but 
the syntax is  function-name open-paren parameter-list close-paren, and 
the stuff in the parens would be a tuple if it were purely data, except 
not quite a tuple because some items are pairs (name and value), but it 
winds up being neither a tuple, nor a list, nor a dict, but instead a 
complex structure related to code execution :)  Actually, this sounds 
quite similar to

     value = expr except (
         Exception1: default1,
         Exception2: default2,
         Exception3: default3,

except that to get the pairing aspect of some parameters for a function call, you use = instead of :, and instead of a named function it has an expression and a keyword.

Not being an expert parser generator, I don't know if the () could be 
made optional if there is only one exception-list, but that would also 
remove one of the benefits some folks might perceive with using this 
syntax, and would also make the analogy with function call syntax a 
little less comparable.

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